Spatial light modulators together with adapted projection systems of so-called “maskless” lithographic systems replace reticles of conventional lithographic systems to provide for imaging onto light-sensitive substrates patterns that are not limited by the physical boundaries or particular patterns of the reticles. Instead of imaging static patterns within the reticles, the maskless lithographic systems image dynamic patterns of focused spots that are individually switched between on and off states while being translated across the substrates.
The spatial light modulators include an array of elements that are individually addressable for functioning as light switches that control a spatial distribution of light. The associated projection system focuses light regulated by each of the elements into a pattern of focused spots, which is relayed (or imaged directly) onto the light-sensitive substrates. Typically, the projection system forms a magnified image of each of the individually addressable elements on corresponding microlenses of a microlens array, and the individual microlenses of the microlens array concentrate the light from the individual addressable elements through the focused spots.
Although both the focusing of the microlenses of the microlens array and the magnification of the projection system result in the focused spots being spaced apart, the pattern of focused spots can be relatively oriented and translated for selectively illuminating every point on the substrate. Typically, the pattern of focused includes multiple rows of focused spots and the rows are oriented at a slight angle to a direction of translation with respect to the substrate so that successive rows of the focused spots collectively sweep whole areas of the substrate. The resolution at which a pattern can be traced onto the substrate relates to the size of the focused spots.
The size of the focused spots can be reduced by increasing the numerical apertures of the microlenses of the microlens array. Increasing the numerical apertures of the lenses correspondingly reduces depth of focus. In addition, changing the numerical apertures of the lenses involves major changes and subsequent realignment of the components of the projection system.